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Posts: 2
Reply with quote  #1 
Back Story:
We live on Big Island, HI. and probably have one of the highest electrical rates in the country.
Monthly electrical bills calculated out to about $0.42/kWh on bills pushing $200/month.
We are retired and on fixed incomes.

About 5 years ago, we had 16 - 280w PV panels installed in a net-metering grid-tie system on our home.
It proved to be a bit short of our average electrical usage.
We ran out of optimal roof area for more PV system.
Plus, new old style net metering agreements have ended.
We were lucky enough to be grandfathered in, though.
So a few months ago, we had a solar hot water system installed.
That put us into a moderate power over production condition, which is what we wanted, a steady minimum grid tie monthly charge with a yearly over production credit to help offset it.

We are on the windward side of the island.
We get more rain and clouds than the leeward side.
Because of electrical costs we had disconnected our spa because it used too much power to operate.
The new household SHW system almost covers the spa's power usage.
It is an old [2006] Baja 250 gal tub.
Baja was an original and major hot tub mfgr. for many years, but is now out of business, but the tub still works just fine.
Spare parts are available and reasonable.
It probably has a long service life left in it.
The spa was being run on a 120v extension cord.
We had it directly wired to 120v recently.

The project:
I want to eliminate electrical water heating for the spa by add a YouTube described type of water heating panel on the roof of the spa's pergola, then pipe water from the spa to the panel, and back using a 120v circulation pump.
The tub's heater is 120v powered.

It seems to me that I should be able to rewire the spa's heater to the circulation pump, removing it from the system.
That should put the SHW system under the control of the tub's automatic heat controls.
Then, tap the cool-water-out line in before the heater location with the hot-water-in line taped in behind it.
That should not require the complete installation of a complete water heating system with controls, keeping the tub's temperature controls operational.
Back-flow preventers and other refinements will probably still be necessary.

Does this seem workable, or am I off in La-La Land?

I have found nothing that describes installing less than a completely new heating and circulatory system plus its controls, or without any controls.


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Posts: 3,058
Reply with quote  #2 
No youre not in lala land and it shouldn't be that difficult. You can make a spa heater out of a coil of black irrigation hose or even dark garden hose. Or you can buy solar pool/spa heaters designed for the purpose.

Though I haven't done it that way I think your idea would work. You would however need a snap disk switch in the collector to prevent the pump from coming on at night and actually cooling the spa. One which comes on at 120F and off at 110 works well. A differential thermostat is better but also more expensive.

Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay

Posts: 2
Reply with quote  #3 
Thanks for your response stmbtwle.

the basic idea for the spa's SWH panel that I'm envisioning is illustrated here:

We're at 19° to 20° N latitude.
The panel could be tilted ~20°.
The tilt should be more efficient.
But, I think a flat mount should work OK.

The tub is presently variably programmable between 80°F and 104°F, with 92° to 94° as the normally set standard temp range.

The panel would be sized to about 100% of the surface area of the tub.

I like the idea of an enclosed panel with glazing, Using copper pipe in a serpentine configuration rather than using headers, or a coil of copper or plastic tubing.
Some designs us a reflective surface inside the panel.
I'm thinking that it is probably a waste.  
It should be finished in black on the interior surfaces.

I will want to insulate all exposed piping to and from the panel.

A pressure relief valve on the panel would be a good thing to have in case of overheating.

A back-flow preventer might or might not be advisable - ??

Thanks, I had not thought about a panel temperature cutoff switch for the lower night temps.

I'm thinking it should be able to hold a variable tub temp from 90° to 96° and cease water flow when the panel is no longer producing enough heat.
Thoughts on making a snap disk switch decision?


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Posts: 3,058
Reply with quote  #4 
If the panel is lower than the tub and you rely on convection, you might not need a cut-off switch.  However if you are using a pump you will.  As the tub cools during the evening the control will turn the pump on... if the panel has already cooled you will get cold water.  

Copper pipe is very good but expensive, and you will need fins (not in the photo) to transfer the heat to the pipe.  IMO overkill for the low temperatures of a hot tub.  Plastic pipe or even poly irrigation tubing has been used.  I would start with a hundred feed of poly tubing coiled up on a piece of insulation board.  You can get more sophisticated later, if need be.

Can't speak for your climate but I think your proposed collector size is way too big.  I used a 2'x4' ARETHA for my tub, but even then I had WAY too much heat in the summer and had to rig up a "heat dump".  But then I'm in Florida.

Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay

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Posts: 143
Reply with quote  #5 
Originally Posted by voyager
Thanks for your response stmbtwle.

the basic idea for the spa's SWH panel that I'm envisioning is illustrated here:

http://www.spa-daddy.com/metals.php" href="https://duckduckgo.com/?q=copper%20not%20compatible%20spa%20chemicals+site:www.spa-daddy.com" target="_blank" rel="nofollow">[com]http://www.spa-daddy.com/metals.php
" data-src="//external-content.duckduckgo.com/ip3/www.spa-daddy.com.ico">https://www.spa-daddy.com
Problems with Copper, Iron, and Manganese Metals Metals - such as copper, iron, and manganese - can discolor the pool water, or can even stain your spa or pool surfaces, if the problem gets ignored. When metals decide to "hang out" in excess, they will color your water.

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